Small-Signal Model-Based Stability Analysis of Grid-Following/Grid-Forming Hybrid System

As the penetration rate of renewable energy continues to rise, the proportion of grid-following (GFL) inverters based on phase-locked loop synchronization is steadily increasing. Under weak grid conditions, increased equivalent impedance at the grid connection point and reduced short-circuit ratio make GFL inverters more susceptible to disturbances, limiting their voltage/frequency support capabilities. In contrast, grid-forming (GFM) inverters actively regulate voltage and frequency, enhancing dynamic performance in weak grids through equivalent inertia and damping support. Hybrid operation combining GFM units controlled by Virtual Synchronous Generators (VSG) with GFL units employing PQ control can achieve complementary advantages in voltage/frequency support and power tracking efficiency. This enhances system support capability while maintaining operational economy. However, the dynamic interaction between these two control systems may induce new oscillation issues, potentially reducing system stability margins. Therefore, this paper constructs a full-order small-signal model of the GFL/GFM hybrid grid-connected system. By integrating the characteristic root locus method with the participation factor method, it analyzes the influence patterns of system and control parameter variations on key eigenvalues. This approach identifies critical parameter thresholds and determines the dominant state variables influencing each oscillation mode. Finally, time-domain simulations and RT-LAB hardware-in-the-loop experiments cross-validate critical dynamic responses—including common coupling point voltage, inverter frequency, and output power—to validate the constructed model and analytical conclusions. These findings provide theoretical foundations for capacity configuration, parameter tuning, and controller optimization in GFL/GFM systems, thereby enhancing stability under small disturbances.